FIELD OF THE INVENTION
The invention relates to an apparatus for protecting the surface of freshly printed sheets, which are deposited stacked on one another to form a sheet pile, the respective sheets having applied thereto by an applicator a separating-agent layer formed of solid particles of separating agent.
Heretofore known apparatuses of this general type, such as a dust applicator device in accordance with the German Patent Document DE 38 19 203 A1, blow separating-agent particles in the form of a powder onto a freshly printed surface of a respective sheet with the aid of compressed air.
As indispensible as a layer of separating agent formed by a powder often is, it can, nevertheless, have a deleterious effect on a final product passing through a printing process. One example is the urgent recommendation that powdered printed products be sanitized before being lined with foil. Any possible scratches in the printed image, for example, caused by joggers in delivery pile apparatuses in cooperation with the powder, remain irreparable.
Types of powder that are supposed to come loose under the influence of the moisture of the printing ink have indeed already been offered. However, tests have demonstrated, for example, in W. Walenski, "Die Bestaubung im Offsetdruck" (Powder Application in Offset Printing), Z. Druckindustrie (Printing Industry Journal), No. 21, Oct. 3, 1993, pp. 21-24, that suitable loosening does not occur even after what must certainly be called an adequate length of time.